The invention relates to a method for operating an internal combustion engine including cylinders with pistons delimiting combustion chambers to which fuel and fresh gas are supplied and from which combustion gases are discharged via controllable inlet and outlet valves.
In a reciprocating-piston internal combustion engine, a combustion chamber, in which fuel is burnt, is delimited by a longitudinally movable piston in each cylinder. Oxygen-containing fresh gas for the combustion of fuel is supplied via intake ports of the cylinders, while the combustion exhaust gases are discharged via exhaust ports. The ports are controlled by intake valves and exhaust valves which are actuated by a valve operating mechanism to carry out the cyclic charge exchange.
In a spark ignition engine, the fuel mix which is formed in the combustion chamber is ignited by the ignition spark of a cylinder spark plug projecting into the combustion chamber. An internal combustion engine operating mode known as compression ignition or chamber ignition is also known for initiating the combustion of the fuel. This operating mode offers the possibility of fuel combustion for driving the internal combustion engine with a good efficiency and a low tendency to form nitrogen-containing gases. DE 198 52 552 C2 discloses a method for operating a four-stroke internal combustion engine, in which combustion exhaust gases are retained in the combustion chamber and, as a result of the increase in temperature of the fuel/air mix, during the compression stroke of the next working cycle of the respective cylinder, a temperature of the compressed volume at which the mix is ignited is reached. Combustion exhaust gases can be retained in the cylinder by means of a valve closure overlap of the control times of the gas exchange valves. The known method seeks to control the valve closure overlap as a function of engine speed and load by means of an exhaust-gas throttle valve which is equally effective for all the cylinders of the engine.
It is also known that compression ignition, that is a fuel combustion method with spontaneous ignition of homogenous fuel/air mix, is carried out only in certain part-load ranges of the internal combustion engine because of certain limitations for this operating mode. DE 199 23 413 A1 describes a method in which the compression ignition is carried out under steady state engine operating conditions in a narrow part-load range of the engine performance map, whereas spark ignition initiated combustion is provided for higher engine loads. To change the operating mode, the known method provides for a change between valve closure overlap and valve opening overlap of the gas exchange valve control times by means of displacement of a camshaft. The camshaft actuating the valve operating mechanism for the gas exchange valves, is switchable, by means of a switching means changing the cam shape for valve opening overlap (spark ignition cycle operation) and a cam shape for exhaust gas retention in the compression ignition mode.
U.S. Pat. No. 6,336,436 B1 discloses a method for changing the operating mode of an internal combustion engine between external ignition and compression ignition, wherein the retention of exhaust gas during the compression ignition mode is realized by changing the control times of the gas exchange valves over the course of internal exhaust-gas recirculation. In this case, in the compression ignition mode, the closure time of the exhaust valve is shifted in the “late” direction compared to the external ignition mode. At the same time, the opening time of the intake valve is advanced, resulting in an opening overlap of the valve control times. The change in the control times of the gas exchange valves with a change in the operating mode is alternatively achieved in the known arrangement by a variably settable mechanism or by a camshaft having a plurality of cam profiles for the forced control of the respective valves. The known method provides for a changeover between the operating modes with a defined transition operating procedure, in which stratified mix formation is provided for by direct injection of fuel during the compression stroke. In the known method, a stable change between the combustion methods is provided for by the transition mode extending over a plurality of working cycles during a change between compression ignition and external ignition. This requires a long transition between the operating mode, since otherwise knocking and severe running irregularities in the internal combustion engine may ensue.
It is the object of the present invention to provide a method for operating an internal combustion engine which changes between an external ignition operating mode and a compression ignition operating mode wherein deteriorations in the overall efficiency of the internal combustion engine, in particular during a change in the operating modes, are avoided by structurally simple means.